This invention relates to an improvement in a power interrupting device of the type including a main interrupter, i.e. circuit breaker, connected to a power source and branch interrupters which are connected to the main interrupter through a current limiting circuit which, in turn, includes a permanent fuse and a parallel resistor.
A conventional power interrupting device of this type is as shown in FIG. 1. In the power interrupting device, the resistance of a parallel resistor R is so selected that very little current flows in the resistor R. During normal operation, current flows through a main interrupter M, primarily through a permanent fuse PF, and through branch interupters F1, F2 . . . and Fn to respective loads. Therefore, when a short-circuit problem occurs in the circuit of any one of the branch interrupters, the permanent fuse PF forming a part of the current limiting circuit becomes extremely high in resistance owing to the high problem current Ipros, thus, starting current limitation. As a result, the value of the problem current is limited by the permanent fuse PF to a limit current peak value Ipfc as shown in FIG. 3. In this case, as the current continues to flow the resistance of the permanent fuse PF becomes higher than that of the parallel resistor R. Therefore, the problem current flows through both the parallel resistor R and the permanent fuse PF, so that a current Ipfr flows as a steady state current. Accordingly, if the current Ipfr flowing in the current limiting circuit is set to 10 to 20 KA, then a main interrupter M and branch interrupters F.sub.i having an interruption capacity of slightly less than the current limit of 10 to 20 KA can be used as those effectively having an interruption capacity of 200 KA as shown in FIG. 2. Since the tripping time of the main interrupter M is delayed, selective interruption can be carried out, and the selective interruption can be effected for large currents.
In the conventional device, the complete interruption time of the main interrupter M must be longer than that of the branch interrupter Fn and shorter than the allowable current flow time of the current limiting circuit (or the permanent fuse PF) and the parallel resistor R. The complete interruption time of an ordinary wiring interrupter is very short, however, and does not provide sufficient time for the selective interruption. Accordingly, in order to employ such a wiring interrupter as the main interrupter in the device, the wiring interrupter must be modified to increase the complete interruption time. Because of this difficulty, a special wiring interrupter must be employed.
On the other hand, if an ordinary air circuit breaker is employed, then sufficient time is available for the selective interruption with the branch interrupter Fn to be carried out, because the complete interruption time of the air circuit breaker is longer. However, since the complete interruption time of the air circuit breaker is longer than the allowable current flow time of the current limiting circuit, the air circuit breaker must be modified to decrease its complete interruption time. Accordingly, it is necessary to use a special air circuit breaker in this case.
In either case, it is technically and economically difficult to develop and obtain such a special wiring interrupter or a special air circuit breaker.